This invention relates to footwear, and in particular to an article of footwear with a cushioning system to protect the wearer from impact combined with a stability system to protect the wearer from uncontrolled motion. It is particularly suited for athletic footwear adapted to accommodate the dynamic motions of the leg, ankle, and foot when walking, running, biking, jumping, turning, and so on. Accordingly, to illustrate the principles of the inventive concepts, it will be described in terms of athletic shoes such as, but not limited to, running, training, walking, and court shoes.
The gait cycle is the repetitive sequence of events that occur during walking or running. Taking heel contact of one foot as the starting event, the stance phase starts with heel contact and ends with toe-off, and the swing phase starts with toe-off and ends with the next heel contact. The stance phase encompasses the period of contact between the foot or footwear and the ground. The swing phase creates the distance traveled during each step.
Throughout the gait cycle, the foot, ankle, and leg anatomy undergo a complex series of three-dimensional motions ultimately governed by the physics of upright bipedal gait. At heel strike, the foot flexes slightly (pronation) to absorb energy and cushion impact. By toe-off, the foot has stiffened (suppination) to push tire body forward. Pronation and suppination have protective and functional benefits. Pronation, for example, cushions the body from impact—but over-pronation can promote certain tendon and knee injuries, among other problems. Suppination provides a rigid platform for push-off—but over-suppination can promote stress fractures and twisted ankles, among other problems. The alternation between pronation and suppination represents an elegant natural solution to the paradoxical “design goals” underlying the role of the foot in weight bearing, locomotion, and equilibrium. The anatomical details are beyond the scope of this discussion and well known in the science of biomechanics.
Shoes are functional extensions of the feet. A shoe supplements the natural mechanisms of the foot to augment its ability to achieve efficient propulsion and protect the body from injury. Just as the foot faces apparently contradictory “design goals,” so too do shoes. An ideal shoe should provide cushioning and shock absorption to protect the wearer. Too much softness, however, can yield a shoe with insufficient foot and ankle stability, potentially contributing to injuries from over-pronation, over-suppination, or excessive foot motion (twisted ankles, say). An ideal shoe should somehow manage to mimic the behavior of the foot, combining softness at impact and stiffness at push-off, while also providing support throughout the gait cycle. Taking inspiration from the foot itself, an ideal shoe should dynamically control the transition from cushion to rebound in both the lateral (side-to-side) and longitudinal (toe-to-heel) dimensions of the shoe. Unfortunately, these objectives are not adequately addressed in conventional shoes, which typically have foot-supporting, sole units that behave monolithically relative to certain foot features of the foot anatomy and do not allow for the natural movement of such anatomy.